One of the major problems with such a process is an efficient and economic manner of heat supply for oil shale retorting. Various solid and gaseous heat carriers have been proposed for retorting oil shale. Substantial dilution of the product vapours and gases from retorting occurs if the heat carrier is hot flue gas, generated for example by combustion of spent shale. Hot recycle gases such as hydrogen, beside having the above drawback, must be heated in a separate furnace using additional fuel and hence the overall thermal efficiency is decreased. Steam as a sole heat carrier is also thermally inefficient because of its high condensation heat.
Solid heat carriers have none of the above drawbacks but good mixing with raw shale must be provided in order to achieve a reasonable rate of heat transfer from the heat carrier solids to the raw shale particles. The TOSCO II process, for example, uses ceramic balls as the heat carrier which deliver their sensible heat to oil shale particles in a rotary retort. The balls must be separated from the smaller spent shale particles after retorting, usually by screening. The balls are then lifted by a mechanical elevator and heated by combustion gases, produced from burning external fuel, in a co-current moving bed heater. The raw shale particles are preheated in order to improve the thermal efficiency of the process and to minimize the expensive recirculation rate of the balls, and preheating is done in a series of dilute phase lift pipe heaters by hot gases leaving the ball heater.
The Lurgi process also uses a solid heat carrier in the form of particulate shale solids heated in a dilute phase lift pipe by partial combustion of residual carbonaceous matter. However, the residence time of solids in the pipe is short and usually only a small fraction of the fuel value of the spent shale is recovered and transferred to the heat carrier solids. Consequently, a large amount of recirculated heat carrier solids is needed to provide the necessary heat for the raw shale retorting. This can cause difficulty in controlling the quality and quantity of the oil product. Before entering the fluidized bed or moving bed retort, the two streams of solids are intensively mixed in a screw type mixer, and so the operation requires reasonable strengths of particles. For the abovementioned reasons, raw shale is also preheated by hot flue gases.